Oxyanion hole

Oxyanion hole is a structural feature found in enzymes that stabilizes the transition state of a substrate during catalysis. It is typically a pocket or groove in the enzyme where an oxygen atom of the substrate can form hydrogen bonds with the enzyme, thus stabilizing the negative charge on the oxygen atom. This stabilization of the transition state lowers the activation energy of the reaction, increasing the rate of the reaction.

Structure and Function

The oxyanion hole is typically formed by the backbone amino acids of the enzyme, often glycine or cysteine. These amino acids have small side chains, allowing the oxygen atom of the substrate to come into close proximity with the backbone amide groups. The amide groups can then form hydrogen bonds with the oxygen atom, stabilizing the negative charge.

The oxyanion hole plays a crucial role in the mechanism of many enzymes, including serine proteases, thiolase, and carboxypeptidase. In these enzymes, the oxyanion hole stabilizes the transition state of the substrate, facilitating the breakdown or formation of covalent bonds.

Examples

One of the most well-studied examples of an oxyanion hole is in the serine proteases, a family of enzymes that includes trypsin, chymotrypsin, and elastase. In these enzymes, the oxyanion hole is formed by two glycine residues, and it stabilizes the transition state of the peptide bond being cleaved.

Another example is thiolase, an enzyme involved in fatty acid metabolism. In thiolase, the oxyanion hole is formed by a cysteine and an adjacent amino acid, and it stabilizes the transition state of the thioester bond being cleaved.

See also

• Enzyme catalysis
• Transition state theory
• Active site